Significantly more U.S. children have a neurodevelopmental or mental health disability than did a decade ago, according to new research.

Disabilities that impair a child's day-to-day living have risen 16 percent, with the greatest increase seen in richer families, according to the study. Conditions such as autism or attention-deficit/hyperactivity disorder appear to lie behind the increase, experts said.

But the surveys of parents in 2001-'02 and 2009-'10 also revealed some good news: The rate of disability due to physical conditions went down, according to the study, which is scheduled for presentation Sunday at the Pediatric Academic Societies' annual meeting in Washington D.C. Data and conclusions presented at meetings are typically considered preliminary until published in a peer-reviewed medical journal.

"This may mean there are differences in people getting early access to care," said study lead author Dr. Amy Houtrow, vice chairwoman of pediatric rehabilitation medicine at Children's Hospital of Pittsburgh. For example, medications for children with juvenile idiopathic arthritis, a potentially debilitating inflammatory arthritis, have improved significantly in recent years, she said.

"For some conditions, it may be that medical care has improved so much that children may have a diagnosis but not a disability," she said, adding that this particular example is from what she has seen in her practice, not from the study data.

For the study, Houtrow and colleagues reviewed data from two National Health Interview Surveys conducted by the U.S. Centers for Disease Control and Prevention (CDC). They included more than 102,000 parents of children from infancy through 17 years of age.

Parents were asked if their children had any limitations in play or activity, received special education services, needed help with personal care, had difficulty walking without supports, had trouble with memory or had any other limitation.

"It's not enough to just have something like ADHD," she said. "You have to be limited somehow by that diagnosis."

The researchers found that nearly 6 million children were considered disabled at the end of the study. Children living in poverty had the highest rates of disability, although poor children didn't experience the largest increases in the incidence of disability during the study period.

Families with incomes 300 percent above the federal poverty level -- around $66,000 for a family of four -- had a 28 percent increase in children with disabilities. Families whose income levels exceeded the poverty level by 400 percent -- about $88,000 -- saw a 24 percent increase in the number of children with disabilities.

Houtrow said it wasn't clear exactly why this was the case, and the researchers suspect increases in neurodevelopmental disorders may be behind the rise.

In children under 6 years old, the trend was most evident, with almost double the rate of neurodevelopmental disorders -- 36 cases per 1,000 children up from 19 a decade earlier.

The increased prevalence of autism spectrum disorders is likely one of the explanations, said Dr. Andrew Adesman, chief of developmental and behavioral pediatrics at the Steven and Alexandra Cohen Children's Medical Center of New York, in New Hyde Park.

Autism spectrum disorders involve impaired communication, social interactions and repetitive behaviors, and can range from mild, as in Asperger's syndrome, to full-blown autism. The CDC estimates that one in 88 children now has a form of autism.

"Even though the study found some differences in disability rates for different socioeconomic status, I would urge any parent who has a concern about their children to discuss it with their child's pediatrician," Adesman said.

Houtrow agreed. "The condition your child has matters, and how they function in their regular life matters," she said. "If they're having trouble doing things that other children do, reach out to health professionals or to community resources to optimize your child's life. We can help children adapt or get accommodations for them."

Houtrow said the overall rise in neurodevelopmental disorders suggests that there may be changes in what is considered socially acceptable.